Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow dye-donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the donor from the back-side to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent dye-receiving layer of the receiver element. The density of the dye forming the image on the receiver can be affected by the amount of dye transferred, which in turn is affected by the amount of dye in the dye layer, the heat the dye layer attains, and the length of time for which the heat is maintained at any given spot on the donor layer.
U.S. Pat. No. 5,096,874 describes a thermal heat-transfer recording method having a dye-diffusion coefficient in the receiving layer of at least 8.3×10−11 cm2s−1 (5×10−9 cm2/min) at 120° C. The saturated transfer ratio of the dye from the dye layer of the heat transfer sheet to the receiving layer of the image-receiving sheet was 40% or more at 120° C. at a printing speed of 33.3 msec/line when the dye layer of the heat transfer sheet was 1 μm thick and the receiving layer of the image-receiving sheet was 6 μm thick. A reflective color density of approximately 1.0 was achieved. U.S. Pat. No. 5,096,874 discloses that the dye diffusion coefficient and the dye transfer ratio can be optimized at any energy level to obtain a high-density image.
U.S. Pat. No. 5,256,622, discloses the use of high viscosity polymers as binders in a dye-donor layer. U.S. Pat. No. 5,256,622 discloses that both ethyl cellulose ether and cellulose acetate proprionate (CAP) are equally adequate as dye-donor layer binders, as long as their intrinsic viscosity is at least 1.6. The print speeds exemplified are slow print speeds of 4 msec/line or greater.
In both of the above references, slow print speeds were used, that is, print speeds of 4 msec/line or greater. At such print speeds, a different dye transfer efficiency is needed than at high printing speeds to effect printing of high-density images.
High printing speeds are less than 4 msec/line, for example, 2 msec/line or less. At high printing speeds, the print head undergoes heat on/off cycles very rapidly. This generated heat must be driven through the dye-donor support assemblage very rapidly to effect the dye transfer from the donor to the receiver. Each layer in the donor can act as an insulator, slowing down the heat transfer through the layers of the donor to the receiver. Because of the short heat application time, any reduction in heat transfer efficiency results in a lower effective temperature in the donor layer during printing, which can result in a lower transferred dye density. It is known to overcome the low print density associated with shorter line times by increasing the printhead voltage, increasing the dye density in the dye-donor layer, or a combination thereof. Applying higher print head voltages can decrease the lifetime of the thermal print head, and requires a higher power supply, both of which increase cost. Increasing the dye density in the dye-donor layer increases costs, as well as increasing the chance of unwanted dye transfer, such as during storage of a dye-donor element.
There is a need in the art for a means of increasing print speed while maintaining or increasing print density, such as by increased dye transfer efficiency, and maintaining or reducing power to the print head.